The invention relates to a method for conveying a flexible thread by means of a pressurized gas, particularly for inserting a weft thread into the weaving shed of a weaving machine, by utilizing an injector of the type comprising a chamber connected to a source of the pressurized gas, a first channel extending from said chamber for a primary gas flow, a second channel merging with said first channel, in which the thread to be conveyed is supplied together with a secondary air flow, from any known source and a third channel in which both flows after have been joined are combined into a single gas flow taking along the thread to be conveyed.
In the method as known up till this moment and which are applied in the modern pneumatic weaving machines generally injectors are used in which the third channel, also indicated by "mixing tube", has a constant cross-sectional area.
With a view to an as economic air consumption as possible and therefore a low air consumption the diameter of the mixing tube is generally minimized. A small mixing tube diameter moreover has the advantage that the path of movement followed by the conveyed thread will deviate relatively little from a predetermined average conveying path so that the injector will present a thread with large certainty within a relatively narrowly limited area at the entrance of the weaving shed of the weaving machine.
The invention aims at improving the method as performed up till now such that with a given pressure and a given air consumption the force imparted by the flowing gas to the thread to be conveyed in the conveying direction is increased and thereby the effectiveness of weft thread insertion is improved.
According to the invention it is assumed that the force F imparted by the flowing gas to the thread may be considered to be in direct proportion with
(a) the length along which the thread is in contact with the flowing gas; PA1 (b) the density of the flowing gas and PA1 (c) the square, at least an essentially higher exponent than one, of the velocity of the flowing gas. PA1 I. applying an injector which only permits the generation of a subsonic flow of the combined conveying gas flow and PA1 II. applying an injector which permits, dependent on the pressure used, establishing a supersonic flow of the combined conveying gas flow.
An obvious idea would be to increase the mixing tube length. However, in a cylindrical mixing tube the velocity of the flowing gas, as seen in the direction of the axis, is not the same everywhere.
Therewith there has to be distinguished between
In the first case, like in the second case, as long as the pressure used is too low to actually arrive at a supersonic flow, a pressure decrease will occur in the conveying direction under the influence of the friction experienced by the flowing gas from the (cylindrical) mixing tube wall which simultaneously produces a density decrease. With a stationary flow, as a like mass quantity flows through each cross-section, the velocity in the cylindrical mixing tube has to be inversely proportional to the density. This implies that the velocity is maximum at the exit end of the tube.
If the feed pressure is increased the velocity initially will increase until at a predetermined value of pressure the speed of sound is reached at the end of the mixing tube. If the feed pressure is still further increased this velocity cannot further increase but only the density will increase.
It follows from the above given consideration of the factors determining the value of the force imparted to a thread, that the largest contribution to force is given at the exit end of the mixing tube. A prolongation of the mixing tube will have increasingly less effect since the contribution is added at the side where the velocity is minimum. At the same time, with an equal pressure drop, less gas will be permitted to flow through the tube so that the density along the full length will be lower than may be realized with a shorter tube. Thereby a lower force contribution per unit of length will occur.
The invention now proposes to use in case of conveying a thread in a subsonic gas flow an injector in which the channel for the combined gas flow is constructed such that the ratio of the cross-section and the mass quantity flow as seen in the flow direction increases such that in each point of the flow path through the mixing tube the loss of density as a result of the friction is, at least approximately, compensated for by the larger cross-section.
This means that the velocity remains constant along the full length of the third channel (mixing tube) and that it has become possible to reach the maximum value (i.e. the speed of sound) along the full length of the third channel. The force imparted by the combined gas flow to the thread increases correspondingly for the force is proportional to the square of the velocity, which has been increased as a result of the measure according to the invention, whereas the density has only decreased linearly.
It is to be noted that the application of the inventive idea leads to mixing tube constructions in which the increase of cross-section per unit of length is relatively small. If expressed in degrees of conical angle, the degree in which a tube constructed according to the invention "conically widens" will vary, dependent on the feed pressure used and on the tube length, between a fraction of one degree to maximally the order of one degree.
An injector constructed according to the invention therewith clearly differs from known injector types having conically widening mixing tubes in which an essentially larger conical angle (5.degree. and more) of the mixing tube wall is used.
In the second case, with a sufficiently high feed pressure, supersonic velocities are achieved in the primary gas flow. In that case, the injector used has, upstream of the merging point of the first and the second channel, a restriction followed by a certain increase in cross-section. Such an injector is e.g. known per se from the Dutch Pat. No. 144.672. Also with said known injector, however, a cylindrical mixing tube is used. The velocity occurring at the throat (this is the merging point of the first and the second channel) therein should be sufficiently strongly supersonic in order to achieve the velocity after mixing with the secondary air flow sucked along with the thread is still supersonic. Thereafter the velocity will quickly decrease as a result of the friction along the cylindrical mixing tube wall. This means that the supersonic flow can be maintained in the known supersonic injector only through a very limited length whereafter the flow through a shock shifts to a subsonic flow. If the initial velocity, after the mixing range, e.g. is 1.5 times the speed of sound, this length will be maximally 10 to 15 times the mixing tube diameter and at twice the speed of sound this will be 20 to 30 times the mixing tube diameter. Realising still higher velocities does not seem to be very realistic since the pressure of the conveying gas supply necessary therefore very quickly increases above the values of 7 to 8 bar as used at the moment.
The improvement aimed at by the invention is achieved in case of a thread conveyance in a supersonic gas flow by using an injector having a third channel constructed such that a velocity decrease which is imminent due to the friction, is compensated for, at least approximately, by a gradually increasing cross-section of said channel.
The measure according to the invention permits using an injector in which a moderate supersonic velocity (to about twice the speed of sound) may be realized which then, contrary to the known construction, may be maintained through a much larger length, particularly along the full length of the third channel. Thereby the force imparted to the thread to be conveyed, which is proportional to the square of the velocity of the conveying gas, is considerably increased.
It also applies to the second case that the application of the inventive measure leads to a third channel (mixing tube) having a conical angle which not exceeds the order of one degree.
The invention likewise relates to an injector adapted to be used with the method according to the invention.
This injector is of the type comprising a chamber connected to a source of the pressurized gas, a first channel extending from said chamber for a primary gas flow, a second channel merging with said first channel, in which the thread to be conveyed is supplied together with secondary air flow, and a third channel in which both flows after they have been joined are combined into a single gas flow taking along the thread to be conveyed, and is characterized according to the invention in that the third channel has, as seen in the conveying direction, a gradually increasing cross-section, i.e. according to a conical angle between a fraction of one degree and the order of a single degree.
As is remarked above already, the present case particularly deals with injectors having a mixing tube, the (average) cross-section of which is as small as possible. The air consumption then namely is minimal, while a weft thread inserted by an injector having a similar narrow mixing tube has a very good directional stability, i.e. will be presented with ample certainty within a relatively narrowly limited area at the entrance of the weaving shed.
In order to keep the inner circumferential area of the mixing tube and therewith the friction losses of the transport gas flow moving along said surface as small as possible, preferably a mixing tube having a circular cross-section will be used.
According to a further feature of the invention the circular cross-section of the third channel which gradually widens in the transport direction is deformed such at its exit end that the exit cross-section is narrowed at least in one direction, the total cross-sectional area at this point however being not essentially decreased.
By this measure the directional stability of the transported threads is increased, at least in one direction, namely in the direction in which the mixing tube section is narrowed, and is at least as great as with an injector having a comparable air consumption, the third channel of which is cylindrical.
This measure is based on the recognition that the distance whereby the weft thread may maximally deviate relative to the ideal path along the mixing tube axis, in the direction perpendicular to the plane of the warp threads (i.e. perpendicular to the reed movement) constitutes the critical point as to the directional stability of the thread.